System for measuring bod by electrolysis

ABSTRACT

A system for measuring BOD by the electrolysis method has a reaction vessel containing the sample. A container of material for absorbing CO2 is received in the mouth of the vessel and is provided with an adaptor. An electrolysis cell storing the electrolyte is received in the adaptor. A cap is placed on the electrolysis cell with an upper circular covering flange spaced from the walls of the cell thus providing an annular space communicating with the electrolyte to permit venting of hydrogen while minimizing evaporation of the electrolyte. When the level of the electrolyte falls below a predetermined minimum, indicating a low 02 pressure in the vessel, a sensing switch energizes a regulated dc current source to start the electrolysis process to replace the 02 consumed by the sample. The system is insensitive to changes in line voltage and electrolyte strength over a design range. Apertures in the cap member and the cell may be selectively aligned by twisting the cap to equalize the pressure in the cell with atmospheric pressure before starting.

[is], v 3,668,102 1 June'6, 197 2- United States Patent Young [54]SYSTEM FOR MEASURING non BY OTHER PUBLICATIONS ELECTROLYSIS Young et a1.Analytical Chem., Vol. 37, May, 1965 p. 784 [72] Inventor:

James C. Young, Ames, Iowa Primary Examiner-T. Tung Attorney-Dawson,Tilton, Fallon & Lungmus [73] Assignee:

low: State University Research Found: t1on,lnc., Ames, Iowa Aug. l3,1969 21 ApplQNoQ: 849,742

[ ABSTRACT ;A system for measuring BOD by the electrolysis method has a[22] Filed:

reaction vessel containing the sample. A container of material forabsorbing CO, is received in the mouth of the vessel and is Us. mam/195204/] T provided with an adaptor. An electrolysis cell storing the elec-Int Cl trolyte is received in the adaptor. A cap is placed on theelectrolysis cell with an upper circular covering flange spaced from thewalls of the cell thus providing an annular space com- 3 W1, mm 04 1 1[51] [58] Field 01 municating with the electrolyte to permit venting ofhydrogen References Cited while'minimizing evaporation of theelectrolyte. When the level of the electrolyte falls below apredetermined minimum, UNITED STATES PATENTS indicating a low 0 pressurein the vessel, a sensing switch energizes a regulated dc current sourceto start the electrolysis process to replace the 0 consumed by thesample. The system 204/] is insensitive to changes in line voltage andelectrolyte v v strength over a design range. Apertures in the capmember ""204/196 and the cell may be selectively aligned by twisting thecap to 196 4 equalize the pressure in the cell with atmospheric pressurehefore starting.

3,088,905 Poepel et 3,362,900 1/1968 Sabins............

9 Cla ms, 3 Drawing Figures PATENTEDJUH s 1972 SHEET 10F 2 PATENTEDJUH 6m2 SHEET 2 OF 2 QmBmE 52mm QDU SYSTEM FOR MEASURING BOD BY ELECTROLYSISBACKGROUND AND SUMlVIARY The present invention relates to improvementsin the electrolysis method of detemiining the biochemical oxygen demand(BOD) of polluted water. The electrolysis method (sometimes referred toas the Clark method) for the continuous determination of oxygen uptakeof a large inhomogeneous biological sample was first described in anarticle by J. W. Clark published in 1959 as the Engineering ExperimentalStation Bulletin No. 11 of New Mexico State University. The principledescribed was the maintenance of pressure in a closed vessel containingthe sample by replenishing the metabolically utilized oxygen through theelectrolysis of dilute acid. The metabolically produced CO was absorbedwith potassium hydroxide. The internal pressure of the cell wasmonitored by a device which would sense a pressure decrease to actuatethe power supply to the electrolysis cell. A dc current was supplied tothe electrolysis cell; and the on time of the current source, measuredon an elapsed time meter, was a measure of the oxygen consumed by thesample.

From Faradays law, one Faraday is the number of ampereseconds requiredto decompose 1 equivalent weight of a substance. In the case of water,for example, 1 Faraday produces 8,000 mg. of at the positive electrodeper 96,485 ampereseconds. Thus, for a constant current, time is a directmeasure of the 0 production.

Improvements in this technique were described in an article published inANALYTICAL CHEMISTRY, vol. 37, p. 784, (May, 1965) entitled An ImprovedApparatus for Biochemical Oxygen Demand by .l. C. Young, W. Garner, andJ. W. Clark.

The electrical design of the power supply used with this earlier modelwas unsatisfactory. There was no control over the current used forelectrolysis; and frequent manual adjustments were necessary. As aresult, fluctuations in the rate of oxygen production would develop dueto changes in line voltage, changes in the electrolyte concentration dueto evaporation from the cell, changes in electrolyte concentration fromthe electrolytic conversion of water to hydrogen and oxygen, anddifferences in the makeup of new electrolyte solutions. Thus, frequentadjustment of the units was required to maintain a constant current.However, in spite of these sources of error, the BOD of samples of rawsewage and synthetic wastes of strengths approximating raw sewage couldbe measured more precisely than was possible with any other methodavailable.

An article reviewing the development of respirometric methods ofdetermining BOD entitled The Determination of Biochemical Oxygen Demandby Respirometric Methods by H. A. C. Montgomery was published in WATERRESEARCH, Pergamon Press, 1967) vol. 1, pp. 631-662.

The present invention provides still further improvements of theelectrolysis method of determining BOD first described by Clark andlater improved as indicated above. The improvements relate to the designof the electrolysis cell and to the overall system which includes aregulated current source. The cell includes a generally cylindricalouter wall, an intermediate cylindrical wall coaxial with the outer walland having an aperture so that the electrolyte is free to flow in eitherside of the intermediate cylindrical wall. A central tube is providedwithin the intermediate wall; and it extends above the level of theelectrolyte. A cap having a frusto-conical body and a circular upperflange is received at the top of the intermediate cylindrical wall ofthe electrolysis cell; and three electrodes pass through the flange asensing or level electrode, a common (or negative dc) electrode, and apositive dc electrode.

In the structural aspects of the new system, the diameter of thecircular flange of the cell cap is approximately equal to the outerdiameter of the exterior cylindrical wall of the cell. When the cap isplaced on the intermediate cylindrical wall in I sealing engagementtherewith, the circular flange is spaced slightly above the outercylindrical wall of the cell a clearance of 2-3 mm. to provide anannular opening which communicates with the portion of the electrolytein which the negative dc electrode is immersed. Thus, the narrow annularopening permits escape of the hydrogen yet minimizes evaporation of theelectrolyte.

Further, an aperture is provided in the upper seating portion of theinner cylindrical wall of the cell (above the electrolyte); and acorresponding aperture is provided in the frusto-conical wall of thecell cap. When these two apertures are in register, the interior of thevessel containing the sample is in communi' cation with the atmosphereby means of the central tube of the cell in order to equalize thepressure within the vessel to that of the atmosphere as a reference orstarting point.

Still further improvements are made in the overall combination byemploying a regulated dc current source to insure that a constantcurrent always flows when the dc electrodes are energized so that anelapsed time meter, actuated only when the regulated dc source isenergized, provides a very accurate measurement of the consumed 0 As iswell known, the amount of 0 produced is a linear function of the timeintegral of current. With the new power supply design, the electrolyzingcurrent is held within 1% of the desired value for i 50 percent changesin electrolyte strength and i 10 percent changes in line voltage.

A manually-selectable switch is provided for setting the current levelso that the current level times the elapsed time is a measure of theconsumed oxygen. The level-sensing electrode in the cell is coupled to arelay. When the level of electrolyte in the space between theintermediate cylindrical wall and the outer wall falls below apredetermined level (indicated by a reduction in the O pressure in thevessel) the relay switches the current-regulated supply to the dcelectrodes and also actuates the elapsed time meter. The negative dcelectrode serves as a common line for the system. Thus, it is also oneof the ac sensing electrodes obviating the need for an additionalelectrode.

Other features and advantages of the present invention will be apparentto persons skilled in the art from the following detailed description ofa preferred embodiment accompanied by the attached drawing whereinidentical reference numerals will refer to like parts in their variousviews.

THE DRAWING FIG. 1 is a composite drawing showing the improvedelectrolysis cell partially broken away together with an illustration ofthe control cabinet;

FIG. 2 is an exploded perspective view of the improved electrolysis cellwith a portion cut away; and

FIG. 3 is a circuit schematic diagram of the control portion of thesystem.

DETAILED DESCRIPTION Referring then to FIGS. 1 and 2, reference numeral10 generally designates the reaction vessel, 11 denotes the electrolysiscell, and 12 refers to the cabinet for the power and control module. Thereaction vessel 10 is a glass (preferably Pyrex) container having acylindrical side wall 13 and an upper, open neck 14. The samplegenerally designated 15 is contained by the vessel 10; and an air space16 is provided above the surface of the sample 15 within the vessel 10.A magnetic stirrer, diagrammatically shown at S, is located within thevessel 13.

Received in the neck 14 of the vessel 10 is an adaptor generallydesignated by reference numeral 17 the adaptor 17 is also preferablyformed of Pyrex glass.

The adaptor 17 includes an upper, tapered bushing 18, the exteriorsurface of which sealingly engages the interior tapered surface of theneck 14. Depending from the bottom of the bushing 18 is a tube-shapedcontainer 19 having a closed bottom. The container 19 holds a volume ofpotassium hydroxide solution 20 for absorbing the CO metabolicallyproduced by the micro-organisms in the sample. A glass wool wick 21 isplaced in and extends above the upper level of the solution 20. Firstand second apertures 23 and 24 (FIG. 2) are formed in the wall of theadaptor 17 toward the upper end of the container portion 19 and incommunication with the air space 16 of the vessel 13. The apertures 23and 24 permit the CO produced from the sample to enter into thecontainer portion 19 of the adaptor l7 and be absorbed by the potassiumhydroxide solution contained therein. As will be clear from subsequentexplanation, the apertures 23 and 24 also permit the ingress of from theelectrolysis cell into the air space 16 where it is consumed by thesample. Both the exterior and interior surfaces of the glass bushing 18are ground to provide an'air-tight seal respectively with the interiorsurface of the neck 14 and the electrolysis cell 1 1. The interiorsurface of the neck 14 is also ground.

Turning now to the electrolysis cell 11, as seen in both FIGS. 1 and 2,it includes an outer cylindrical wall 26, an intermediate cylindricalwall 27 coaxial with the outer wall 26, and a central tube 28 extendingcoaxially with the walls 26 and 27. The outer annular space defined bythe cylindrical walls 26 and -27 is designated 29a; and the innerannular space between the wall 27 and the tube 28 is designated 29b. Thelower portion of the wall 26 is formed inwardly and converges to theintermediate wall 27; and the bottom of the tube 28 diverges to becomeintegral with the interior surface of the wall 27.

Depending from the lower portion of the outer wall 26 is aninwardly-tapered seating member 30 having an exterior ground surface forseating into the bushing 18 of the adaptor 17. The exterior surface ofthe seating member 30 is correspondingly tapered as seen in FIG. 1.Toward the bottom of the intermediate cylindrical wall 27 there isprovided an aperture 31 so that the annular space 29a communicates withthe annular space 29b. An electrolyte 33 is located in both of theseannular spaces and is free to move through the aperture 31. Thepreferred electrolyte is a 0.3 to 0.5 N solution of sulfuric acid. Theupper portion of theintermediate wall 27 is tapered to a narrowingdimension at the top to define a conical seating surface 27a forreceiving a cap, generally designated by reference numeral. 35, and alsopreferably formed of Pyrex glass. An aperture 34 is formed in theconical portion 27a of the intermediate wall 27.

The cap 35 includes a frusto-conical receptacle 37 for fitting over thecorrespondingly-tapered section 270 of the intermediate wall 27. Theinterior surface of the receptacle 37 is ground, as is theexterior-surface of the tapered portion 27a of the intermediate wall 27to provide an air seal. The cap 35 also ihcludes an upper circularflange 38 integral with the receptacle 37; and it defines threeapertures 39, 40 and 41 for receiving three separate electrodes, aspresently to be described. The outer diameter of the circular flange 38is approximately equal to the outer diameter of the cylindrical sidewall 26; and when the cap 35 is fully seated on the intermediate sidewall 27, there is an annular spacing designated 44 between the lowersurface of the flange 38 and the upper edge of the side wall 26. Theannular space 44 is preferably about 2 to 3 mm. This annular aperturepermits the escape of hydrogen produced in the electrolysis of theelectrolyte 33 yet minimizes'the evaporation of the electrolyte; and ithas been found to be of advantage for long-term, accurate use of thecell, as is normally contemplated.

The receptacle 37 of the cap 35 defines an aperture 37a which may beselectively aligned with or sea] the aperture 34 of the seating portion27a of the intermediate side wall 27 by twisting the cap 35. When thecap 35 is placed on the cell 11 as illustrated in FIG. 1, a chamberdesignated 47 in FIG. 1 is provided. The chamber 47 is defined by thetapered portion 27a of the intermediate side wall 27 and the cap 35. Thechamber 47 communicates by means of the tube 28, the seat- 1 ing portion30 of the cell 11, and the apertures 23 and 24 of the adaptor 17 withthe upper air space 16 of the vessel 13. When the apertures 34 and 37aare aligned, the chamber-47 also communicates with the atmosphere bymeans of the annular spacing 44 to equalize pressure within that chamberwith the atmosphere and bring the level of the electrolyte within theinner annular space 29b to be the same as the level of the electrolytein the annular space 290. When the cap 35 is then rotated so that theaperture 34 and 37a are not aligned, the chamber 47 and the air space 16are sealed from the atmosphere. When 0, is then metabolically consumedby the sample and CO, is given off into the chamber 16, the CO, istransmitted through the apertures 23 and 24 into the interior of thetube container 19 wherein it is absorbed by the potassium hydroxide 20,thus generating a slight vacuum. This reduced pressure is transmitted tothe chamber 47 to bring the electrolyte in the inner annular space 29bto a higher level thereby lowering the level of the electrolyte in theouter annular space 29a.

A first sensing electrode 50 extends through the aperture 39 in thecircular flange 38, and is secured thereto by means of an epoxy cement.The electrode 50 extends to a predetermined level within the outerannular space 29a of the cell 1 l, and it is normally in contact withthe electrolyte. When the CO, in the air space 16 is absorbed by thepotassium hydroxide solution, as already explained, the electrolyte inthe outer annular space reduces its level to break contact with thesensing electrode 50. As will be explained in greater detail within,when this break in contact is made, a dc current is applied toelectrodes 52 and 53 received respectively in the apertures 40 and 41 ofthe circular flange 38 and sealed thereto with epoxy. The electrodes 50,52 and 53 are preferably made of platinum.

The electrode 52 extends through the chamber 47 and into the electrolytein the inner annular space 29b. The electrode 53 contacts theelectrolyte 33 in the outer annular space 29a, as shown. Thus, when theelectrolyte 33 breaks contact with the sensing (or level-detecting)electrode 50, indicative of a reduced pressure in the chamber 47, theelectrolysis process is initiated; and O is produced at the positiveelectrode 52 to increase the pressure within the chamber 47 and at thesame to replenish the supply of O in the air space 16. Hydrogen isproduced at the negative electrode 53 and transmitted to the atmospherevia the annular aperture 44. This process will continue until thepressure within the chamber 47 reduces the level of electrolyte in theinner annular space 2% thereby raising the level of the electrolyte inthe outer annular space 29a to contact the electrode 50. When contactoccurs, electrolysis is terminated. Thus, the level of electrolyte ismaintained within a narrow range, thereby maintaining the O pressurewithin the air space 16 substantially constant.

Turning now to FIG. 3, power leads 56 and 57 are energized by aconventional 60 Hz. ac source, not shown. A fuse 58 and a main switch 59are interposed in the line 56. The primary winding of a firsttransformer 60 is connected to the source by the lines 56 and 57; and asecondary winding of the transformer 60 feeds a full-wave rectifierbridge, generally designated 61, to produce a dc output voltage. Acapacitor 62 filters this dc voltage; and it is fed via a manual ON/OFFswitch 63 to a current-regulated power supply enclosed within the dashedline 64. Since it is contemplated that a plurality of samples will betested at one time, it is preferable that there be only one source of dcvoltage and that the dc voltage be transmitted to a separatecurrent-regulated power supply for each cell in use. The positivepotential of the bridge output is coupled to the collector of an NPNtransistor Q1, the base of which is connected to the negative terminal(which is the system common or ground) of the bridge circuit by means ofa Zener diode Z. A current path is provided between the collector andbase of Q1 by means of a resistor 65 in order to bias Zener Z andprovide a path for base current for transistor Q1. The emitter oftransistor Q] is coupled through a resistor 66 to the emitter of a PNPtransistor Q2. The collector of the transistor Q2 is connected to themovable contact of two-position relay contacts 67a. The normally-openterminal of the contacts 67a is connected to a dummy load resistor 68.The other side of resistor 68 is connected to the system common. Thenormallyclosed contacts 67a are connected in series with a plug-in jack69 and a current meter 70 to the positive electrode 52. Electrode 53 isconnected to the system common and serves as the negative dc electrodewhen the electrolyte is below its predetermined level in the annularspace 29a (i.e., out of contact with the sensing electrode 50). Theelectrode 53 also serves as a common line for the ac source, asdescribed below.

The base of transistor O2 is connected to the emitter of a second PNPtransistor Q3, the collector of which is connected in common with thecollector of transistor Q2. Thus the transistors Q2 and Q3 form aDarlington amplifier. A resistor 70a is connected between the emitter oftransistor Q1 and the base of transistor Q3. The resistor 70a acts incombination with a second resistance as a voltage divider network tomaintain the voltage at the input of the Darlington amplifier at aselected, constant value. The second branch of the voltage dividernetwork comprises a selected resistance value to determine the currentoutput from the regulator circuit 64. That is to say, there are fourseparate branches of resistance, selectable by a switch 71 having itsmovable arm connected directly to the system common and four fixedterminals denoted I-IV. Specifically, a first variable resistor 73 isconnected in series with a fixed resistor 74 between the base oftransistor Q3 and a fixed terminal I of the switch 71. A second variableresistor 75 is connected in series with a fixed resistor 76 between thebase of transistor Q3 and fixed terminal ll of the switch 71. A thirdvariable resistor 77 is connected in series with a fixed resistor 78 toposition III of the switch 71; and a fourth variable resistor 79 isconnected in series with a fixed resistance 80 to fixed terminal IV ofthe switch 71. Each of the variable resistances 73, 75, 77, and 79permits fine adjustment of the output current to a predetermined value;and once an initial adjustment is made, these settings normally are notchanged thereafter.

In operation of the current regulator, the Zener diode Z maintains aconstant voltage at the base of transistor Q1. Since transistor Q1 isbiased in the active region, its base-emitter junction isforward-biased, so the voltage at the emitter of transistor Q1 issubstantially constant for all ranges of current. Thus, the first stageof the current regulator takes the dc output of the bridge circuit 61which varies with the line voltage and converts it to a lower, butconstant value independent of i percent of changes in line voltage.Hence, the voltage across the divider network (including resistor 70aand one of the selected branches connected to the switch 71) maintains aconstant but selectable voltage at the base of transistor Q3. As long asthe transistors Q2 and Q3 are in the active region, the total collectorcurrent is determined by the base current which is set by the voltageacross resistor 70a and emitter resistor 66. As the load changes, thevoltage change across resistor 66 adjusts the base current oftransistors Q2 and Q3 to maintain the load current at a constant value.The total resistance of the branches connected to positions l-IV ofswitch 71 are progressively smaller to increase the voltage acrossresistor 70a as the movable contact of switch 71 is moved to a higherposition. Thus, the output current from the collectors of transistor Q2and O3 is substantially independent of the load value within the designrange. For example, a constant load current can be achieved for changesof i 50 percent in electrolyte strength and t 10 percent changes in linevoltage.

The primary winding of a transformer 84 is connected across the powerlines 56 and 57; and the secondary winding of the transformer 84 isconnected respectively between the system common and the coil of a relay67. The other terminal of the coil of relay 67 is connected directly tothe sensing electrode 50. The previously-described contacts 67a areactuated by the relay 67 that is, when the coil of relay 67 is notenergized. The contacts 670 couple the output of the current regulatordirectly to the positive electrode 52 (via jack 69 and current meter70). On the other hand, when the coil of relay 67 is energized(indicating that the level of electrolyte in the annular space 29a is atleast as high as the predetermined level) the contacts 67a connect theoutput of the current regulator circuit directly to a dummy load(resistor 68) so that the current regulator circuit is always operativeand operating at a stabilized equilibrium temperature for greateraccuracy. The relay 67 is provided with a second set of contacts 67b;and these contacts are connected in series with a switch 85 for couplingthe power lines 56 and 57 to an elapsed time meter 86. The contacts 67bare normally closed. Thus, when the electrolyte falls below itspredetermined level, thereby de-energizing the coil of relay 67, notonly is the current-regulator switched to electrode 52, but the elapsedtime meter 86 begins to measure time. An indicator lamp 87 is connectedbetween the power line 57 and the junction of switch 85 and contacts76b; and when it is lit, it indicates that the current regulator andelapsed time meter circuits are energized. Switch numbers 63 and 85refer respectively to either side of a single double-pole switch.

Returning to FIG. 1, the dial for current meter 70 (which may simple bea multiplication factor) is designated 90 on the face panel of themodule 12; and the scale for the elapsed time meter 86 is shown at 91.The knob for the selector switch 71 is designated 92; and the shaft ofON/OFF switch and elapsed time meter circuit switch 85 is designated 94.The pilot light 87 is also indicated.

Having thus described a preferred embodiment of the improved system forthe determination of BOD by electrolysis, it will be appreciated thatthe operation of the system is rendered more accurate through the use ofa regulated and highly accurate dc power supply which is in operativerelation between the two electrolyzing electrodes 52 and 53 only whenthe electrolyte level is below a predetermined level indicative of thereduced pressure within the vessel containing the sample. The currentoutput of the regulated supply is insensitive tovariations in linevoltage and electrolyte strength. Further, one of the electrolyzingelectrodes (namely, electrode 53) serves as a system common and is usedin combination with the sensing electrode 50 to detect the level of theelectrolyte within the cell. When the electrolyte is at least as high asa predetermined level (established by the tip of electrode 50) in theouter annular space 29a, the electrodes 50 and 53 are energized by an acsource; whereas during the electrolysis stage, the electrodes 52 and 53are energized by a regulated dc source. The setting up of the system isfacilitated by the provision of the aligned holes 34 and 37arespectively in the intermediate cylindrical wall 27 and the cap 35 toequalize the pressure within the vessel with the atmosphere prior tostarting. It will also be appreciated that the circular flange 38 of thecap 35, having substantially the same diameter as the diameter of theexterior wall 26 of the electrolyte cell 11 and being spaced therefromby a small distance of the order of 2-3 mm., greatly minimizes theevaporation of the electrolyte solution 33 while permitting escape ofthe hydrogen gas produced at the electrode 53. In addition, by securingthe electrodes to the cap 35, access to the interior of the cell 11 forcleaning and maintenance is greatly facilitated.

Persons skilled in the art will be able to modify certain of thestructural aspects described and illustrated and to substituteequivalent components while continuing to practice the principles of theinvention; and it is, therefore, intended that all such substitutionsand modifications be covered as they are embraced within the spirit andscope of the appended claims.

What is claimed is:

1. A system for determining the biochemical oxygen demand of a sample,comprising a vessel for containing the sample and having an upper airspace, an electrolysis cell containing an electrolyte in first andsecond chambers, said chambers communicating with each other at a levelbelow the level of said electrolyte, said second chamber providing aclosed space communicating with the air space of said vessel, a sensingelectrode extending in said first chamber to a predetermined level, anegative electrode immersed in said electrolyte in said first chamber ata level below said predetermined level, a positive electrode immersed insaid electrolyte in said second chamber, circuit means responsive to thefalling below said predetermined level in said first chamber by saidelectrolyte and thereby breaking contact with said sensing electrode,and

current-regulated dc source means for energizing said negative andpositive electrodes respectively with a negative and positive dcpotential at a constant, pre-selected current rate only when saidelectrolyte level is below said predetemiined level to produce oxygen atsaid negative electrode, said oxygen being communicated to said airspace of said vessel, and to produce hydrogen at said electrode, said dosource means including rectifier means receiving energy from an acsource and generating a dc voltage, isolation means receiving the outputvoltage of said rectifier means for generating a constant dc voltageindependent of fluctuations in said ac source, and regulator circuitmeans for generating a constant current responsive to the output voltageof said isolation means.

2. The system of claim 1 further comprising circuit means including arelay having a coil connected in circuit with said sensing electrode, acmeans to energize said sensing electrode, said coil and said negativeelectrode with an ac signal, said relay having a normally-closed contactfor coupling said do supply between said positive and negativeelectrodes only when said relay coil is de-energized by saidelectrolytes breaking contact with said sensing electrode.

3. The apparatus of claim 2 further comprising second normally-closedcontacts actuated by said relay, a time mechanism in series with saidsecond relay contacts, and a source for energizing said contacts andsaid timer whereby when said relay is de-energized, said do source isconnected to said positive and negative electrodes and said timersimultaneously.

4. The system of claim 1 wherein said isolation means further comprisesselection means for selection of the output voltage thereof to therebychange the output current of said regulator circuit.

5. The system of claim 1 wherein said regulator circuit means includes acircuit-selectable switch and a plurality of resistor networks connectedin circuit with said switch for selecting different levels of outputcurrent.

6. The system of claim 5 further comprising timing mechanism actuatedonly when said current-regulator means is energized.

7. The system of claim 6 further comprising a current meter in serieswith said current-regulator means and said electrolyte to measure thecurrent therein.

8. ln apparatus for measuring BOD by electrolysis, the combination of avessel containing a sample and provided with a throat and an air spaceabove said sample adjacent said throat, a cell received in said throatand having an outer wall, an intermediate wall and a central tube toprovide first and second spaces respectively between said outer wall andsaid intermediate wall and said intennediate wall and said tube forcontaining an electrolyte, said intermediate wall defining an aperturebelow the electrolyte level to communicate said first and second spaceswith each other, a sensing electrode normally contacting saidelectrolyte in said first space, positive and negative electrodesrespectively in said first and second spaces and immersed in saidelectrolyte, dc source means for energizing said positive and negativeelectrodes responsive to said electrolytes breaking contact with saidsensing electrode, a cap member received on said intermediate wall andcooperating therewith to define a chamber above said electrolyte in saidsecond space, said chamber communicating with the air space in saidvessel via said tube, said intermediate wall defining a second apertureabove the level of said electrolyte, said cap member defining a thirdaperture for selective registration with said second aperture of saidintermediate wall whereby when said second aperture of said intermediatewall and said aperture of said cap are aligned, the air space of saidvessel communicates with the atmosphere to equalize the pressuretherein.

9. The apparatus of claim 8 wherein said outer and intermediate walls ofsaid cell are cylindrical and said intermediate wall is provided with aninwardly-tapered upper portion, and

wherein said cap member includes a frusto-conical receptacle for fittingover said tapered portion of said intermediate wall in sealingengagement therewith, and a circular flange integral with saidreceptacle, the periphery of said flange extending substantially to theouter diameter of said outer wall and spaced therefrom of the order ofthree millimeters to provide a narrow annular opening therebetween forpermitting hydrogen generated at said negative electrode in said firstspace to escape while minimizing the evaporation of said electrolytefrom said first space.

1. A system for determining the biochemical oxygen demand of a sample,comprising a vessel for containing the sample and having an upper airspace, an electrolysis cell containing an electrolyte in first andsecond chambers, said chambers communicating with each other at a levelbelow the level of said electrolyte, said second chamber providing aclosed space communicating with the air space of said vessel, a sensingelectrode extending in said first chamber to a predetermined level, anegative electrode immersed in said electrolyte in said first chamber ata level below said predetermined level, a positive electrode immersed insaid electrolyte in said second chamber, circuit means responsive to thefalling below said predetermined level in said first chamber by saidelectrolyte and thereby breaking contact with said sensing electrode,and current-regulated dc source means for energizing said negative andpositive electrodes respectively with a negative and positive dcpotential at a constant, pre-selected current rate only when saidelectrolyte level is below said predetermined level to produce oxygen atsaid negative electrode, said oxygen being communicated to said airspace of said vessel, and to produce hydrogen at said electrode, said dcsource means including rectifier means receiving energy from an acsource and generating a dc voltage, isolation means receiving the outputvoltage of said rectifier means for generating a constant dc voltageindependent of fluctuations in said ac source, and regulator circuitmeans for generating a constant current responsive to the output voltageof said isolation means.
 2. The system of claim 1 further comprisingcircuit means including a relay having a coil connected in circuit withsaid sensing electrode, ac means to energize said sensing electrode,said coil and said negative electrode with an ac signal, said relayhaving a normally-closed contact for coupling said dc supply betweensaid positive and negative electrodes only when said relay coil isde-energized by said electrolyte''s breaking contact with said sensingelectrode.
 3. The apparatus of claim 2 further comprising secondnormally-closed contacts actuated by said relay, a time mechanism inseries with said second relay contacts, and a source for energizing saidcontacts and said timer whereby when said relay is de-energized, said dcsource is connected to said positive and negative electrodes and saidtimer simultaneously.
 4. The system of claim 1 wherein said isolationmeans further comprises selection means for selection of the outputvoltage thereof to thereby change the output current of said regulatorcircuit.
 5. The system of claim 1 wherein said regulator circuit meansincludes a circuit-selectable switch and a plurality of resistornetworks connected in circuit with said switch for selecting differentlevels of output current.
 6. The system of claim 5 further comprisingtiming mechanism actuated only when said current-regulator means isenergized.
 7. The system of claim 6 further comprising a current meterin series with said current-regulator means and said electrolyte tomeasure the current therein.
 8. In apparatus for measuring BOD byelectrolysis, the combination of a vessel containing a sample andprovided with a throat and an air space above said sample adjacent saidthroat, a cell received in said throat and having an outer wall, anintermediate wall and a central tube to provide first and second spacesrespectively between said outer wall and said intermediate wall and saidintermediate wall and said tube for containing an electrolyte, saidintermediate wall defining an aperture below the electrolyte level tocommunicate said first and second spaces with each other, a sensingelectrode normally contacting said electrolyte in said first space,positive and negative electrodes respectively in said first and secondspaces and immersed in said electrolyte, dc source means for energizingsaid positive and negative electrodes responsive to said electrolyte''sbreaking contact with said sensing electrode, a cap member received onsaid intermediate wall and cooperating therewith to define a chamberabove said electrolyte in said second space, said chamber communicatingwith the air space in said vessel via said tube, said intermediate walldefining a second aperture above the level of said electrolyte, said capmember defining a third aperture for selective registration with saidsecond aperture of said intermediate wall whereby when said secondaperture of said intermediate wall and said aperture of said cap arealigned, the air space of said vessel communicates with the atmosphereto equalize the pressure therein.
 9. The apparatus of claim 8 whereinsaid outer and intermediate walls of said cell are cylindrical and saidintermeDiate wall is provided with an inwardly-tapered upper portion,and wherein said cap member includes a frusto-conical receptacle forfitting over said tapered portion of said intermediate wall in sealingengagement therewith, and a circular flange integral with saidreceptacle, the periphery of said flange extending substantially to theouter diameter of said outer wall and spaced therefrom of the order ofthree millimeters to provide a narrow annular opening therebetween forpermitting hydrogen generated at said negative electrode in said firstspace to escape while minimizing the evaporation of said electrolytefrom said first space.